1. Field of the Invention
The invention relates to an operational amplifier (OPAMP), and more particularly to a telescopic operational amplifier.
2. Description of the Related Art
With semiconductor technology development, analog to digital converters (ADCs) operate at lower voltage. Thus, the reference buffer of the ADC is required to operate at lower voltage. When speed and resolution of an ADC are higher, the reference buffer of the ADC must have high bandwidth.
FIG. 1 is a schematic diagram of a conventional reference buffer. Reference buffer 100 comprises a differential OPAMP 110, resistors R1 and R2. Resistors R1 and R2 generally comprise the same impedance for obtaining a compromise between gain and bandwidth of differential OPAMP 110.
FIG. 2 is a schematic diagram of the differential OPAMP 110 of FIG. 1. Differential OPAMP 110 is a two stage structure for obtaining high gain and low output impedance. However, in high speed design, the second stage of differential OPAMP 110 usually consumes a large amount of current in order to push the second pole far away. Additionally, because the feedback factor of differential OPAMP 110 is approximately 0.5, the closed loop bandwidth of differential OPAMP 110 is half compared to the unity gain buffer topology.